The instructions in that tutorial will install MongoDB from the regular Ubuntu repository. In order to provide a stable environment, Ubuntu and other distros release with a specific versions of software. Often they will provide backported security patches and bugfixes, but they will not ship new major releases.

You can install a more recent version of MongoDB using upstream’s repository:

sudo apt-key adv --keyserver hkp://keyserver.ubuntu.com:80 --recv 7F0CEB10
echo 'deb http://downloads-distro.mongodb.org/repo/ubuntu-upstart dist 10gen' | sudo tee /etc/apt/sources.list.d/mongodb.list
sudo apt-get update
sudo apt-get install mongodb-org

Installing the mongodb-org packages will automatically remove the Ubuntu mongodb-server ones.